The envelope glycoprotein (env) of HIV-1 is synthesized as the precursor gp160 which is processed into the proteins gp120 and gp41 and which remain noncovalently associated on the outer surface of HIV. The gp120 is the surface glycoprotein responsible for binding to CD4, the cellular receptor for HIV-1. The gp41 is transmembrane glycoprotein involved in membrane fusion. The mechanism of viral nucleocapsid entry into the host cell is believed to be mediated by gp120 binding to CD4 this leads to conformational changes in gp41 which activate its fusogenic activity. Towards understanding this process in molecular detail, we have expressed in E.coli part of the extracelluar domain, residues 27 to 149, of the related SIV gp41. The expressed protein forms insoluble "inclusion bodies". We have developed methods for the solubilization, purification, folding and crystallization of the protein. The biophysical properties of the protein indicated the protein is an all helical rod-like trimer with a molecular mass of 47 kDa. To improve the physical properties of the protein we substituted cysteines at positions 86 and 92 for alanine residues. Both the wild type and mutant protein have been crystallized. The crystals are suitable for high resolution structure determination. In addition, protein biosynthetically labeled with selenomethione has been crystallized. Structure determination of the gp41 by NMR is also well underway in the laboratory of Marius Clore (NIDDK). To support this work we have supplied protein uniformly labeled with various combinations of the stable isotopes: D, C-13 and N-15. Current work is aimed at producing the HIV-1 gp41 with and without the N-terminal fusion peptide. We also aim to study the interaction of gp41 with gp120 using biochemical and biophysical methods.